1. Field of the Invention
The present invention relates to a permanent-magnet synchronous motor in which permanent field magnets are provided in a rotor.
2. Description of the Related Art
Generally, concentrated-winding permanent-magnet synchronous motors are constructed such that 3n teeth of a stator core are disposed at a uniform pitch where n is a positive integer, a stator coil is formed by Y-connecting three phases of coil each wound independently onto the teeth, and 2n permanent magnets are disposed so as to face the stator core. In other words, concentrated-winding permanent-magnet synchronous motors have a 2n-pole permanent magnetic field disposed relative to 3n teeth.
Because a concentrated winding method is adopted in conventional permanent-magnet synchronous motors constructed in this manner, inductance is increased by adjacent teeth forming opposite poles, facilitating the action of demagnetizing fields on the permanent magnets. Thus, with the magnetic poles generated by the stator coil and the magnetic poles of the permanent magnets opposing each other, one problem has been that portions of the magnetic fields generated by the stator coil enter the permanent magnets and act on the permanent magnets as demagnetizing fields, demagnetizing the permanent magnets.
In order to overcome problems of this kind, a permanent-magnet synchronous motors has been proposed as an improvement in Japanese Patent No. 3076006, for example, in which a space between the teeth is reduced to allow magnetic fields generated by the stator coil to flow to adjacent teeth instead of entering the permanent magnets.
In this conventional permanent-magnet synchronous motor proposed as an improvement in Japanese Patent No. 3076006, a space (La) between an adjacent pair of tooth tip portions and an air gap (Lg) between a stator core and a rotor, are constructed so as to satisfy an expression 0.3 Lg<La≦2.0 Lg. Furthermore, the space between the teeth (La) is set between 0.3 mm and 0.4 mm.
Because the space (La) between the adjacent pair of tooth tip portions is equal to or less than 2.0 times the air gap (Lg), the flow of demagnetizing magnetic flux to the rotor (the permanent magnets) can be suppressed. As a result, it is claimed that even if the magnetic poles generated by the stator coil and the magnetic poles of the permanent magnets oppose each other, demagnetizing fields are less likely to act on the permanent magnets, enabling improvement in the strength of the permanent magnets against demagnetization.
Because the space between the adjacent pair of tooth tip portions in the conventional concentrated-winding permanent-magnet synchronous motor proposed as an improvement, as explained above, is constructed so as to be narrow, magnetic flux leakage between the teeth is increased, increasing inductance in the stator coil. The increase in inductance in the stator coil gives rise to an increase in a time constant of the stator coil, increasing a phase difference between a drive voltage applied to the stator coil and a driving current that actually flows through the stator coil. As a result, there have been problems of deterioration in the heavy load characteristics wherein surplus voltage during heavy loads is reduced, lowering the rotational frequency.